MUSCULAR CONTRACTION IN TISSUE-CULTURES. 201 



appearance of coarse fibrils passing from cell to cell, as described by Benda (1902) 

 and McGill (1907a, 1909). An interesting picture was obtained by fixing a culture 

 with 10 per cent nitric acid and later staining it (fig. 14). In this case the edge of 

 the cells became coagulated in such a manner as to imitate the appearance described 

 by McGill (1909, fig. 15) as "heavy, elastic fibers" and "the more delicate con- 

 nective tissue network." However, in this case the delicate connective-tissue 

 network was formed by coagulation of traces of the rapidly withdrawing inter- 

 cellular bridges and the elastic fibers by the fixation of the curled edges of cells 

 just beginning to retract. Such behavior is not without precedent in the fixation 

 of living material. The work of Levi (1916a), Lewis and Robertson (1916), Lewis 

 and Lewis (1917c), and Chambers (1917), etc., has demonstrated that the actual 

 threads of the mitotic spindle are not present in the living cell. Cowdry (1914) 

 states that the Nissl substance does not exist in the living nerve cell. Marinesco 

 (1912), Mott (1912), and Lewis and Lewis (19126) claim that neurofibrils are caused 

 by the fixation of the nerve fiber. This by no means infers that myofibrils are 

 artefacts; it merely claims that the substance formed by the cell during differentia- 

 tion does not necessarily exist as threads in the living cell, but that it assumes 

 this form upon the coagulation of the cytoplasm. At least it can be demonstrated 

 in these smooth-muscle cells of the amnion that threads (myofibrils) appeared 

 upon the fixation of the living cell, where none existed previously. 



So far it has proved impossible to fix the area of contraction as it appeared in 

 the living cell, even in cases where the fixing solution was injected through an 

 opening in the ring of vaseline directly upon the contracted cell. As soon as the 

 fixative touched the cell the folds disappeared, although the protoplasm remained 

 thicker and more concentrated in this region. Coincident with this, more or less 

 straight, coarse fibrils were formed (fig. 13). After fixation the appearance of 

 many of the cells was such that it might easily lead to an erroneous conception of 

 the phenomenon of contraction; namely, that it had been caused by a shortening 

 and thickening of the myofibrils in such a manner as to concentrate the protoplasm 

 in this region. When fixed contracted muscle cells were stained (fig. 13), the area 

 of contraction, in which folds had been present before fixation, appeared much 

 the same as that structure termed by McGill (1909) the "contraction node." Con- 

 cerning this, Miss McGill states : 



"During contraction more changes take place in smooth muscle than can be attributed 

 to morphological causes, such as thickening of the myofibrils, etc. At the contraction 

 nodes the staining reaction would indicate that there is a marked chemical reaction taking 

 place also." 



This view coincides with that expressed above, that there is a center of active 

 change. 



HEART-MUSCLE. 



Burrows (1912) first described the growth and contraction of the muscular 

 cells arising from pieces of embryonic chick heart explanted in plasma. According 

 to Dr. Burrows, these cells do not contain cross striations, although they undergo 

 rhythmical contraction for 24 to 96 hours. In the new growth there could be 



